Run a job against MSRV

From the comment above the job being changed here, it seems that this
job was supposed to be run with MSRV Rust but it uses the latest stable
instead. Let's make it work as it was intended.

This would hopefully catch us unintentionally bumping the MSRV.

.github/workflows/build.yml

@@ -345,6 +345,9 @@ jobs:
   testcfail:
     name: testcfail
     runs-on: ubuntu-latest
+    env:
+      RUSTFLAGS: -D warnings
+      MSRV: 1.87.0
     defaults:
       run:
         working-directory: cfail
@@ -374,7 +377,9 @@ jobs:
             ${{ runner.OS }}-build-
 
       - name: Install Rust
-        uses: dtolnay/rust-toolchain@stable
+        uses: dtolnay/rust-toolchain@master
+        with:
+          toolchain: ${{ env.MSRV }}
 
       - name: Run cargo
         run: cargo run